When operating fuel cell stacks in fuel cell systems, in particular in fuel cell systems in motor vehicles, it is necessary to be able to determine the current density distribution in the fuel cell stacks. Various methods of measuring a current density distribution in fuel cell stacks are known. Appropriate methods for this purpose are described in the publication xe2x80x9cIn-situ methods for the determination of current distributions in PEM-fuel cells,xe2x80x9d by Jxc3xcrgen Stumper, Ballard Power Systems Inc., 9000 Glenlyon Parkway, Burnaby, BC, Canada V5J 5J9, Electrochimica Acta, Vol. 43, 1998 and in the publication xe2x80x9cA printed circuit board approach to measuring current distribution in a fuel cell,xe2x80x9d by S. J. C. Cleghorn, C. R. Derouin, M. S. Wilson, S. Gottesfeld, Los Alamos, Journal of Electrochemistry, July 1998.
These conventional methods for measuring the current density distribution in fuel cell stacks have the following disadvantages. First of all, each of these measurement methods requires an intervention (manipulation) in the fuel cell stack. Measurement reactions or feedbacks and corruptions of the measurement results resulting from this manipulation of the fuel cell stack thus cannot be excluded. Furthermore, it is necessary to define, prior to performing the measurement and the manipulation of the fuel cell stack, the point in the cell at which the corresponding current density distribution is to be measured. With this conventional method, the wiring complexity for the measurement increases with an increasing resolution, thus limiting its acceptable scope. Due to the complexity of the conventional measurement methods and the manipulation that is required for this purpose on the fuel cell stack, the conventional measurement methods are suitable only for research and development of fuel cell stacks, but cannot be used for a continuous measurement during operation or in service intervals. A further disadvantage of the conventional measurement methods is that the variation of the current density distribution along the fuel cell stack, that is to say the variation from cell to cell, cannot be measured when using these methods.
It is accordingly an object of the invention to provide a method for determining the current density distribution in fuel cell stacks, which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and through the use of which the current density distribution in fuel cell stacks can be determined, across the fuel cell cross section, at any desired point in the fuel cell stack.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for determining a current density distribution in a fuel cell stack, the method includes the steps of:
providing a current flow through the fuel cell stack such that the current flow generates a magnetic field surrounding the fuel cell stack; and
determining the current density distribution in the fuel cell stack from the magnetic field surrounding the fuel cell stack.
According to a preferred mode of the method according to the invention, the magnetic field which surrounds a fuel cell stack through which a current is flowing is advantageously measured at a number of points, and the current density distribution across the fuel cell cross section in the interior of the stack is then deduced on the basis of this measurement.
The method according to the invention for determining the current density distribution in fuel cell stacks by measuring the magnetic field which surrounds the stack in this case has the advantage that no change (manipulation) in the fuel cell stack itself is required. Furthermore, measurement reactions from the measurement technique on the current density distribution can be virtually completely avoided. In addition, the current density distribution can be measured in any desired cell in the stack, without needing to define one cell in advance. Furthermore, it is possible to measure the variation of the current density distribution along the stack (from cell to cell). Thus, in comparison to the conventional methods, the measurement method according to the invention allows a greater measurement accuracy due to the greater resolution, while at the same time considerably reducing the costs for each measurement process.
A preferred mode of the method according to the invention includes the steps of positioning at least one sensor at given positions outside the fuel cell stack for detecting an x-component of a magnetic flux density, a y-component of a magnetic flux density, and a z-component of a magnetic flux density, with x, y, and z indicating axes in a Cartesian coordinate system; determining the given positions of the at least one sensor with respect to the fuel cell stack; calculating current density values as a function of respective positions in the fuel cell stack from values of the magnetic flux density and the given positions associated therewith; and performing the calculating step by using a Maxwell""s equation defining a magnetic field strength, and by using a material equation defining a relationship between the magnetic field strength and the magnetic flux density.
Another mode of the method according to the invention includes the step of sequentially recording measurement points by moving a single flux density sensor to the measurement points in order to sequentially measure a magnetic flux density at the measurement points and by moving a further sensor to the measurement points in order to determine a position of the single flux density sensor.
Yet another mode of the method according to the invention includes the step of recording measurement points in parallel by using a plurality of sensors for measuring a magnetic flux density and for determining respective positions such that all measurement values are recorded simultaneously.
A further mode of the method according to the invention includes the step of recording measurement points with a plurality of sensors in parallel and additionally recording measurement points sequentially by recording a set of measurement values in parallel and by subsequently repositioning the plurality of sensors and recording a further set of measurement values.
Another mode of the method according to the invention includes the step of identifying a reference point and using an algorithm for counting steps in a positioning device operating with fixed increments for positioning the at least one sensor.
Yet another mode of the method according to the invention includes the step of indicating a position of the at least one sensor relative to the reference point.
A further mode of the method according to the invention includes the steps of positioning a plurality of sensors spatially fixed with respect to one another; and performing a parallel measurement with the plurality of sensors.
Yet a further mode of the method according to the invention includes the steps of providing the plurality of sensors on a common mount such that the plurality of sensors are spatially fixed with respect to one another; and moving the plurality of sensors jointly along an x-direction defining a main direction of the fuel cell stack.
Another mode of the method according to the invention includes the step of varying a distance between the at least one sensor for detecting a magnetic flux density and the fuel cell stack in order to match a measurement range of the at least one sensor to a magnetic flux density outside the fuel cell stack.
Yet another mode of the method according to the invention includes the step of positioning a plurality of sensors for detecting a magnetic flux density in a given plane.
Another mode of the method according to the invention includes the step of positioning a plurality of sensors for detecting a magnetic flux density in a given plane such that an x-axis defines a main axis of the fuel cell stack and such that the x-axis is orthogonal with respect to the given plane and such that the given plane and the x-axis define an intersection point.
A further mode of the method according to the invention includes the step of evaluating the x-component of the magnetic flux density in order to identify points at which the current density distribution in the fuel cell stack changes, wherein the x-component of the magnetic flux density is directed along a main direction of the fuel cell stack.
Another mode of the method according to the invention includes the step of using sensors each configured to measure three magnetic flux density components including the x-component of a magnetic flux density, the y-component of a magnetic flux density, and the z-component of a magnetic flux density.
Yet another mode of the method according to the invention includes the step of using sensors each configured to measure only one magnetic flux density component selected from the group consisting of the x-component of a magnetic flux density, the y-component of a magnetic flux density, and the z-component of a magnetic flux density.
Another mode of the method according to the invention includes the steps of using at least a first sensor for detecting the x-component of a magnetic flux density; and using at least a second sensor for detecting the y-component and the z-component of a magnetic flux density.
A further mode of the method according to the invention includes the step of acquiring more measurement values than necessary for a desired resolution of the current density distribution in the fuel cell stack.
Another mode of the method according to the invention includes the steps of providing an equation system for the current density distribution in the fuel cell stack; and using an iterative calculation method for calculating the equation system for the current density distribution.
Yet another mode of the method according to the invention includes the step of deducing the current density distribution in the fuel cell stack by comparing measurement values of the magnetic field surrounding the fuel cell stack with magnetic fields of fuel cell stacks having known current density distributions.
A further mode of the method according to the invention includes the steps of providing an equation system for the current density distribution in the fuel cell stack; using an iterative calculation method for calculating the equation system for the current density distribution; and additionally deducing the current density distribution in the fuel cell stack by comparing measurement values of the magnetic field surrounding the fuel cell stack with magnetic fields of fuel cell stacks having known current density distributions.
Another mode of the method according to the invention includes providing an equation system for the current density distribution in the fuel cell stack; using a Monte Carlo algorithm for calculating current density values in order to solve the equation system by performing the steps of using an assumed current density distribution for providing a calculated magnetic field resulting from the assumed current density distribution; comparing the calculated magnetic field with a measured magnetic field; correcting the assumed current density distribution in a subsequent iteration loop, such that a difference between the calculated magnetic field and the measured magnetic field is reduced; and terminating the Monte Carlo algorithm when a correlation between the calculated magnetic field and the measured magnetic field reaches a given correlation strength, and using the assumed current density distribution as a result.
A further mode of the method according to the invention includes the step of increasing a resolution of the current density distribution calculated from measurement results by increasing a number of measurement values for the magnetic flux density and the given positions of the at least one sensor.
Another mode of the method according to the invention includes the steps of measuring an earth""s magnetic field prior to detecting a magnetic flux density outside the fuel cell stack; and subtracting the earth""s magnetic field from the magnetic flux density detected outside the fuel cell stack.
Yet another mode of the method according to the invention includes the steps of modulating the current flow through the fuel cell stack with a given low frequency; and suppressing an influence of a quasi-static earth""s magnetic field on a measurement result by using a hardware filter and/or a software filter for flux density measurements.
A further mode of the method according to the invention includes the steps of supplying all measurement values of the magnetic field surrounding the fuel cell stack to an electronic measurement processing system; automatically calculating and storing the current density distribution in the fuel cell stack; and comparing the current density distribution with previous measurement results.
Yet a further mode of the method according to the invention includes the steps of supplying all measurement values of the magnetic field surrounding the fuel cell stack, of the given positions of the at least one sensor and of the current flow through the fuel cell stack to an electronic measurement processing system; automatically calculating and storing the current density distribution in the fuel cell stack; and comparing the current density distribution with previous measurement results.
Another mode of the method according to the invention includes the step of measuring a magnetic field strength of the magnetic field generated by the current flow through the fuel cell stack.
A further mode of the method according to the invention includes the steps of performing a first measurement by scanning the fuel cell stack entirely with a coarse resolution; and performing a second measurement by scanning critical areas of the fuel cell stack with a fine resolution.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining the current density distribution in a fuel cell stack, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.